Protein synthesis in the endoplasmic reticulum (ER) can be dramatically upregulated under stress conditions. To avoid jamming of protein translocation and folding, the ER senses early indicators of such stress conditions and responds with the so-called unfolded protein response (UPR). This is an ancient signal transduction pathway that leads to the rapid replenishment of ER chaperones and other folding factors and avoids energy- requiring repair mechanisms before committing to apoptosis. The UPR is critical to sustain cell growth and development and to combat disease and abiotic stress. Interestingly, the efficacy of the UPR varies largely among individuals of the same species, but the underlying molecular causes are unknown. To initiate the UPR, yeast relies heavily on the action of a conserved ER stress sensor, Ire1p. During the course of evolution, the suite of UPR sensors has expanded to accommodate more specific responses in a multicellular context. The basic activation mechanisms and general function of the ER stress sensors are largely known from in vitro studies and cell culture analyses. However, how the UPR regulators work coordinately to sustain healthy cell growth and development with a minimum of energy costs is unknown. We wish to address this fundamental question in Arabidopsis thaliana, because of the conservation of plant and metazoan UPRs, the vast availability of genetic diversity and genomics resources for this model species, and the relevance of the plant kingdom as a source for renewable energy, food, and materials. The immediate goals of this proposal are 1) to define the molecular determinants underlying intra-specific UPR variability using natural populations with broad genetic diversity, 2) to understand the mechanisms that control homeostasis among the various UPR pathways in vivo, and 3) to define non-conventional mechanisms that modulate ER stress responses in intact organisms. To achieve our goals, we will pursue our genome-wide studies to characterize UPR diversity as well as our functional genomics analyses to define the mechanisms for homeostasis of the UPR signaling pathways in vivo. We will also use advanced next-generation sequencing strategies to define post-transcriptional modulation of the UPR. Our results will lead to a broad and deep understanding of the complexity of the UPR signaling network during ER stress in the context of complex multicellular organisms. Adding plants as an evolutionarily distinct and tractable model for the study of the UPR in complex organisms is also important because it will allow comparing and contrasting plant, yeast, and animal UPRs, and thus will provide significant insights into these systems, adding to the fundamental knowledge of eukaryotic cell biology at large. Our results will not only enhance our understanding of human growth and disease, they will also permit the development of drugs in a tractable multicellular model, and contribute to our knowledge of limiting factors in agricultural processes and plant biotechnology designed to sustain food security on earth.